1. Field of the Invention
The present invention relates to a display device having an image capture function.
2. Related Art
A liquid crystal display device includes an array substrate on which signal lines, scan lines, and pixel TFTs are disposed in horizontal and vertical directions and a drive circuit for driving the signal lines and the scan lines. A processing technology for forming a part of the drive circuit on the array substrate has been commercialized by the recent advance and development of integrated circuit technologies. Accordingly, liquid crystal display devices can be made light, small and thin in their entirety, which permits the liquid crystal display devices to be widely used as a display device for various mobile equipment such as mobile phones, notebook type computers, and the like.
Incidentally, there has been proposed a display device provided with an image capture function by disposing an intimate contact type area sensor for capturing an image on an array substrate (for example, Japanese Laid-Open Patent Application Publications Nos. 2001-292276 and 2001-339640).
Conventional display devices having this type of the image capture function capture an image by changing the amount of charge of a capacitor connected to a sensor according to a quantity of light received by the sensor and by detecting a voltage at both the ends of the capacitor.
However, since a current flowing through the sensor is weak, it is difficult to accurately detect the change of the voltages at both the ends of the capacitor caused by the current, which results in a significant error in measurement. Accordingly, there is a likelihood in which noise appear in a captured image.
Further, when an SRAM and a buffer circuit are connected to the capacitor to detect the voltage at both the ends of the capacitor, whether a logic level is at “0” or at “1” is determined depending upon whether the voltage exceeds the threshold value voltage of a transistor constituting the SRAM and the buffer circuit. However, since the threshold value voltage of the transistor is varied, there is a possibility that a determination criterion of “0” and “1” is varied. Further, since the current flowing through the sensor is also varied, there is a possibility that the determination criterion of “0” and “1” is also varied.